Aromatic polycarbonate, further referred to herein as polycarbonate, is a widely used raw material in many different manufacturing sectors. Due to the hardness and transparency of the material, it can be applied in applications as diverse as automotive windows and optical lenses. It is believed that the demand for polycarbonate will increase significantly in the coming years, requiring improvement in the production of polycarbonate, particularly in terms of efficiency and environmental impact.
Several processes for the production of polycarbonate are known. For instance, a process including reacting phosgene and 2,2-bis(4-hydroxyphenyl)propane (commonly known as bisphenolacetone, or BPA) under phase transfer conditions is applied on an industrial scale. However, this process has the inherent drawbacks of employing the toxic component phosgene and creating chloride containing waste streams.
A different process that does not require the use of phosgene is based on the transesterification of bisphenolacetone with dialkyl carbonate or diaryl carbonate.
The dialkyl carbonate has the disadvantage that in the transesterification with bisphenolacetone, it is not reactive enough, so that polymeric polycarbonate cannot be formed. Furthermore, the alkyl alcohol that is liberated is not used in any other part of the process for producing polycarbonate. Recycling of the alkyl alcohol to the dialkyl carbonate production however will require substantial purification.
The use of a diaryl carbonate, in particular diphenyl carbonate (DPC), has the advantage that it is reactive enough to form polymeric polycarbonate. Furthermore, phenol is liberated in the reaction of the diphenyl carbonate with bisphenolacetone to form polycarbonate, for instance as described in U.S. Pat. No. 5,589,564. This phenol may in turn be recycled to the production of bisphenolacetone or diphenyl carbonate, for which it is a main raw material.
The use of the liberated phenol for the manufacture of diphenyl carbonate requires a substantial purification, as described in U.S. Pat. No. 5,747,609. A more efficient approach is thus to employ the liberated phenol for the production of bisphenolacetone, as described in U.S. Pat. No. 6,277,945 without further purification.
All of the above-described processes have in common that large amounts of separate raw materials need to be produced, transported and stored, or that several large production units must be combined on a single production site, which is usually not feasible for environmental and economical reasons.
The above process, which combines the production of bisphenolacetone and polycarbonate, needs as raw materials acetone, phenol and diphenyl carbonate. The latter two are solids at ambient temperature, which implies that if large amounts of these materials are transported, a number of problems arise that affect both the safety and economics of the overall process.
Diphenyl carbonate has a melting point of 78–79° C., which makes a transport in molten state impracticable, as most standard transport vessels for liquid materials are not equipped to maintain a temperature above 70° C. However, safe transport and handling of the molten product (e.g. with minimal waste from tank washings) requires maintaining the product at a temperature of about 15 to 20° C. above the melting point. Transport of liquid materials at such temperature would also require a large amount of energy, and could lead to problems with solidifying material if not properly handled. Only a limited number of vessels are even capable of such proper handling at these temperatures, all with rather smaller tank sizes.
The transport of diphenyl carbonate in the solid state on the other hand requires the diphenyl carbonate to be solidified after its production. This is usually accomplished by cooling the diphenyl carbonate, and by forming it into suitable particles, which can then be bagged and transported as solid material. Generally, cooling and particle formation require large and complicated equipment such as cooling bands and/or prill towers. Such equipment unnecessarily increases the capital investment, and is also expensive and energy consuming to operate.
The handling and transport of solid diphenyl carbonate have drawbacks common to handling of solids in general. For instance, the solid particles have to have a suitable size and size distribution according to their subsequent application, as otherwise the material may not flow freely due to blocking. This size and size distribution are difficult to maintain, as the particles are prone to sinter upon exposure to even moderately elevated temperature and/or pressure. The particles may also build-up electrostatic charges upon handling, which increases the hazard of explosions and fire. A further problem occurs when reheating the diphenyl carbonate particles to obtain a molten product or a solution. This not only consumes much energy, but also can lead to partial degradation and discoloration of the material due to hot spots.
Additionally, contamination with dust during cooling, crushing or transport is difficult to avoid. This may lead to contamination of the polycarbonate, which is detrimental to properties of polycarbonate products, in particular when used in optical devices.
Hence, the above process for production of polycarbonate leaves ample room for improvement, in particular in view of the way the raw materials are introduced.